1. Field of the Invention
The present invention relates to a compatible optical pickup apparatus and a method for controlling the phases of three beams. More particularly, the present invention relates to a compatible optical pickup apparatus which can simultaneously control focus offset and phases of three beams with respect to discs having different thicknesses, and a method for controlling the phases of the three beams.
2. Description of the Related Art
An optical pickup is used to record or reproduce data with respect to an optical disc and includes a light emitting portion and a light receiving portion. The light emitting portion allows light emitted from a light source to be incident on the disc. The light receiving portion allows light reflected from the disc to be received by a photodetector so that data is reproduced or a servo is performed.
Referring to FIG. 1A, a conventional compatible optical pickup apparatus includes an optical module 103 having a first light source 101 emitting light having a wavelength of about 650 nm and a second light source 102 emitting light having a wavelength of about 780 nm, a light source unit 100 having a grating 104 supported by a holder 105 and dividing light emitted from the first and second light sources 101 and 102 into three beams for performing a tracking servo or focusing servo function, a beam splitter 107 for changing a proceeding path of the light emitted from the first and second light sources 101 and 102, an object lens 113 for focusing light reflected from the beam splitter 107 on first and second discs 110 and 111, and a photodetector unit 115 for detecting light by receiving the light reflected from the first and second discs 110 and 111. A collimating lens 112 for converting incident light into a parallel beam is provided between the beam splitter 107 and the object lens 113.
The first light source 101 is used for the first disc 110 which is relatively thin while the second light source 102 is used for the second disc 111 which is relatively thick. The light emitted from the first and second light sources 101 and 102 is reflected by the beam splitter 107 and proceeds toward the first and second discs 110 and 111. After being reflected from the firs and second discs 110 and 111, the light passes through the beam splitter 107 and is received by the photodetector unit 115. The photodetector unit 115, as shown in FIG. 1B, includes a first photodetector 117 for detecting the light reflected from the first disc 110 and a second photodetector 118 for detecting the light reflected from the second disc 111. The first photodetector 117 has a four-section structure. The second photodetector 118 includes a main photodetector 118a having a four-section structure and a pair of sub-photodetectors 118b and 118c arranged on both sides of the main photodetector 118a. 
The light emitted from the first and second light sources 101 and 102 are diffracted into three beams by the grating 104. A tracking error is detected using the three beams in a differential phase detecting method with respect to the first disc 110 and a three beam method or a push-pull method with respect to the second disc 111. The first photodetector 117 used for the differential phase detector comprises a four-section photodetector. Among a +1st order beam 121, a −1st order beam 122, a 0th order beam 123, which are diffracted by the grating 104, only the 0th order beam 123 is formed on the first photodetector 117. Thus, the +1st order beam 121 and the −1st order beam 122 are not used in detecting track errors of the first disc 110. Since the differential phase method, the three beam method, and the push-pull method are widely known techniques, detailed descriptions thereof are omitted.
According to the above structure, a compact disk (CD) and a digital video disk (DVD) can be compatibly recorded and reproduced using a pickup apparatus. However, during setting for the compatible optical pickup apparatus, an operation to satisfy conditions for reproducing and recording data for both CD and DVD is required. Since the CD and DVD have different data recording properties or are different in physical structure, satisfying all of the respective conditions is needed.
For this purpose, it is common to first set the conditions for the DVD and then the conditions for the CD. To obtain a recording/reproducing performance with respect to a disc, conditions such as photodetector balance (PDB), RF level, focus offset, tracking offset, and phase must be satisfied. The PDB is used to adjust the light reflected from the disc for focusing at the center portion of the photodetector. FIG. 2A shows an S curve which indicates the occurrence of a focus error signal. Preferable focusing is obtained when the S curve passes the zero point 0 such as for a curve S1. When the S curve is shifted and does not pass the zero point 0 such as for the curve S2, a focus offset is generated. Since focusing is not normally performed when the focusing offset is generated, the S curve must be adjusted to pass the zero point 0.
Since the PDB and the focus offset are the most important conditions to meet, the following descriptions will focus on these conditions only. The photodetector is moved in x and y directions to adjust the PDB of the DVD while the light source unit 100 is moved in a z direction to adjust the focus offset. Thus, the positions of the light source unit 100 and the photodetector unit 115 for meeting the optimal conditions are fixed. After the setting for the DVD is complete, the conditions for the CD need to be met. However, since the optical conditions for the DVD are already met, the conditions for the CD need to be met without changing the present state of the DVD parameters. Thus, to meet the PDB for the CD, the positions of the light source unit 100 or the photodetector unit 115 must not be changed. Considering the above, to meet the PDB for the CD, the photodetector unit 115 is rotated without changing the position of the photodetector unit 115. Since the first photodetector 117 of the photodetector unit 115 is set to be optimal for the DVD, the photodetector unit 115 is rotated by a predetermined angle θ1 with respect to the first photodetector 117 so as not to change the position of the first photodetector 117.
Next, for the tracking of the CD, a tracking error detection is performed using the three beam method. To accurately detect a tracking error, the phases of three beams must satisfy a predetermined angle, for example, 180°. The grating 104 is rotated to control the phases of three beams. The grating 104 is rotated so as not to change the position of the light source 100. In other words, since the first light source 101 is presently set to satisfy the optimal conditions for the DVD, the holder 105 is rotated by a predetermined angle θ2 with respect to the first light source 101 without changing the position of the first light source 101. When the grating 104 is rotated, as shown in FIGS. 2B and 2C, the phases of the three beams e, f, and g focused on a track 123 of the CD are controlled. In the graph of FIG. 2C, a tracking error signal by the beams e and g is shown and the phase difference between the beams e and g is 180°.
In adjusting the PDB with respect to the CD, although in the above description the photodetector unit 115 is rotated without changing the position of the first photodetector 117, the position of the first photodetector 117 practically changes. Thus, to readjust the PDB with respect to the DVD, an operation to adjust the position of the photodetector unit 115 needs to be performed. After the PDBs with respect to the DVD and the CD are repeated, an optimal PDB for both the DVD and CD can be obtained.
Also, in controlling the phases of three beams with respect to the CD, although in the above description the first light source 101 is not to be moved in the z direction when the holder 105 is rotated, the position of the first light source 101 is practically changed. Thus, to readjust the focus offset for the DVD, an operation to adjust the position of the light source unit 100 must be performed. After the focus offset for the DVD and the phases of three beams for the CD are repeatedly adjusted, optimal conditions can be obtained for both the DVD and CD.
However, in adjusting the PDB of the CD, since the effect by the rotation of the photodetector unit 115 on the PDB of the DVD is small, it is unnecessary to readjust the PDB of the DVD after adjusting the PDB of the CD, or the number of repeated adjustments is small. In contrast, when the holder 105 is rotated to control the phases of three beams for the CD, the rotation of the holder 105 greatly affects the focus offset of the DVD. Thus, since the adjustments need to be repeated to control the focusing offset of the DVD, a lot of time is required and labor efficiency is reduced.